Obtaining Quality In Additive Manufacturing

Additive Manufacturing is entering industrial serial production. Especially in regulated industries such as aviation and medical, the need for internationally accepted standards and proven practices for machine qualification is continuously growing. To meet this demand, Ampower has published a comprehensive approach and best practices to establish a qualified production environment and gives an overview on standardization efforts and published standards.

Additive Manufacturing (AM) technologies have developed to an industrial production technology during past years. As a result of this
technology maturity, the current lack of international standards has become obvious. Therefore, committees ISO TC261 and ASTM F42 on a international level, in close cooperation with VDI on a German national level, are providing a powerful platform to deliver the industry standards needed.

In 2017 the size of the worldwide additive manufacturing market volume was estimated at 2,3 billion USD with a compound annual growth rate of 31% predicted until 2020. To reach these high growth rates year-by-year, increasingly more industrial AM applications need to be exploited. In contrast to Rapid Prototyping applications, Additive Manufacturing use cases require larger lot sizes and will therefore significantly increase total market volume growth. In order to enable and foster the application of AM in industrial scenarios,
there is a strong need for standards.

“Many business cases for Additive Manufacturing with laser beam powderbed fusion technology are in regulated industries such as aviation or medical. However, the current lack of standards and knowledge on AM qualification increases the threshold for companies to adapt the technology. Therefore Ampower partnered with Trumpf to conduct an extensive study and present a comprehensive qualification approach and best practices to establish a qualified production environment.” DR. Eric Wycisk Managing Partner at Ampower.

To avoid competing international standards, ISO TC261 and ASTM F42 joined forces in 2011 and decided to develop standards together. The so called PSDO agreement (“Partner Standards Developing Organization”) provides the organizational frame. The spirit that drives this unique partnership is best described by a slogan from the current Chairman of ISO TC 261 Joerg Lenz “One world – one standard!”. The process to publication of an ISO/ASTM standard is complex and lengthy and starts with a demand typically expressed by industry representatives to either ISO TC261 or ASTM F42. On the German National Level, VDI committees 105.X is driving the development of guidelines for AM. Dr.-Ing. Christian Seidel, an internationally recognized AM expert comments “Since AM standardization is on a very promising path. Especially in 2017 and 2018, the AM community got together and agreed on streamlined procedures to develop the standards needed to foster Additive Manufacturing applications.”

Challenging Requirements in Aviation

The fast adaption of Additive Manufacturing has forced the aircraft and turbine OEMs to develop their own specifications and standards for manufacturing and supplier qualification. In addition to EN/AS 9100 and Nadcap certification, aviation suppliers have to fulfill the detailed and challenging requirements defined by the OEMs.

The qualification documentation designed by the aviation OEMs includes detailed material and process specifications and instructions. The severity of requirements vary depending on part critical and qualification method (part or process qualification). To define these specifications the OEMs are developing deep process knowledge to understand limitations and risks in their respective R&D departments.
Laser powder bed fusion is the most researched AM technology in aviation and therefore on the forefront of the qualification process. The areas of interest for defined requirements are, among others, the process strategy, machine technology, quality insurance, testing intervals, non-destructive testing and personnel requirements.

The defined specifications have to be fulfilled by the in-house production facility or the supplier depending on the chosen supply chain model. Aviation giant Airbus, for example, outsources the Additive Manufacturing to its strategic tier 1 suppliers, that must fulfill the extensive and challenging specifications to receive production approval. In the future the tier 1 supplier may in turn subcontract to further suppliers, while being responsible for the sub-supplier qualification.

A connector from B/E Aerospace Systems GmbH represents one of the first qualified metal parts in aviation for military application. The connector is being produced by AM specialist Robert Hofmann GmbH since 2006. The component is part of the pilot’s oxygen system of an Eurofighter vessel and has to withstand gas pressures of over 600 bar.

The National Aerospace and Defense Contractors Accreditation Program (Nadcap) is a global cooperative accreditation program initiated by and focused on the aviation industry. Through the Performance Review Institute (PRI) the Nadcap program provides independent certification for manufacturing processes. Nadcap was one of the first certification institutions to develop an audit and certification program especially for powder bed fusion technologies. The audit criteria by Nadcap comply closely to established qualification approaches for AM by the medical industry. However, additionally they put a strong focus on the powder handling and recycling procedures.

Qualified Production

A qualified production in regulated environments requires a validated process of which the machine qualification is a significant component. Furthermore, a detailed specification and documentation of each process step ensures reproducible execution. Continuous and gapless reporting guarantees full traceability of all critical process parameters and the material flow.

Machine Qualification

The machine qualification is a series of inspections and tests to objectively verify that all necessary requirements for the final product quality are fulfilled. This includes procedures and documentation for the installation, operation and maintenance of the machine. The requirements are typically derived from industrial standards, manufacturer and customer specifications. Additionally to the standard operational procedures and working instructions required for the general quality management, the machine qualification requires detailed working instructions and standard. Since medical applications have been one of the first regulated industrial AM applications, the methodologies from the Global Harmonization Task Force for machine qualification have been widely accepted in the AM industry.

Specifications and Records

Additionally to the standard operational procedures and working instructions required for the general quality management, the machine qualification requires detailed working instructions and standard forms on each critical process step. Furthermore protocols for production, maintenance and quality assurance have to be established.

Process Validation

In regulated environments, a machine qualification must be performed for each single system in the process chain. In Additive Manufacturing this includes at least the AM machine and heat treatment oven and must be expanded to processes of hot isostatic pressing, milling and others, if relevant to the manufacturing of the final product. If the qualification of each system is successful, the whole process chain can be validated by the “first part qualification” or “first article inspection”, where parameters for production are defined and the final product passes through the complete process chain while complying to all given requirements.

Machine Acceptance

The machine acceptance verifies that the installed system fulfills all requirements defined in the technical specification. The final machine acceptance during the site acceptance test can be combined with the installation and operational qualification.

The machine acceptance is performed to determine, if all requirements from the technical specification are met by the AM system at delivery. Typically the machine acceptance is divided into two separate events. First the factory acceptance test (FAT) at the supplier site has to be passed before the customer allows shipment to his own site. After the machine installation at the customer site, the successfully performed site acceptance test (SAT) shows the conformity of the AM system with the predefined machine requirements.

Promoted by the aviation industry, the German Institute of Standardization (DIN) published “DIN 35224 Welding for aerospace applications – Acceptance inspection of powder bed based laser beam welding machines for additive manufacturing” to unify and regulate the minimum criteria for the acceptance of laser powder bed fusion systems. Within the standard the main system components and characteristics required for inspection and their respective acceptance criteria are defined.

Machine Acceptance Build Job

The machine acceptance build jobs allow the evaluation of required material properties as well as geometrical capabilities and dimensional accuracy. The evaluation should consider the full build volume.

The machine acceptance build job contains test specimens for the most relevant material requirements of the planned product portfolio. This typically includes density, microstructure and tensile properties in different building orientations and positions. For demanding applications additional specimens are added to evaluate fracture toughness or fatigue properties. It is advisable to choose testing methods that facilitate a quick evaluation of results. Especially for fracture mechanical values, such as fatigue or fracture toughness, test methods such as alternating
deformation behavior or Charpy impact testing accelerates the evaluation of the results and reduces testing cost. It is highly recommended to determine the material properties across the full build volume or at least the intended build envelope. The acceptance build job should be manufactured during the FAT and SAT. If time or cost constraints prohibit this, it is recommended to build the individual acceptance job at least during the SAT to evaluate the machine performance.